A cationically photopolymerizable composition has heretofore been used in applications such as an adhesive, a photoresist, a paint, and a sealing material. The cationically photopolymerizable composition is cured by irradiation with an active energy ray, such as UV light or visible light. Accordingly, the composition does not require heating and can be cured in a short time period. An acrylic resin-photoradical curing-type composition or an epoxy resin-photocation curing-type composition has been known as the cationically photopolymerizable composition. When each of those cationically photopolymerizable compositions is irradiated with an active energy ray having a proper wavelength and proper intensity, its curing is completed within from several seconds to several tens of seconds.
Literatures 1 to 5 described below disclose various cationically photopolymerizable compositions.
Literature 1 (JP 2000-109511 A) discloses a cationically photopolymerizable composition containing: a cationically photopolymerizable compound having at least one vinyl ether group per molecule; a cationic photopolymerization initiator; and a weakly basic substance.
Literature 2 (JP 2004-189895 A) discloses a photopolymerizable composition containing a cationically polymerizable compound, a cationic photopolymerization initiator, and a monofunctional vinyl monomer, in which the cationically polymerizable compound is obtained by blending an epoxy resin having two or more glycidyl groups per molecule and an epoxy resin having one glycidyl group per molecule at a ratio of 100:10 to 100:150, and 100 parts by weight of the canonically polymerizable compound is blended with 5 parts by weight to 30 parts by weight of the monofunctional vinyl monomer (see claim 1).
Literature 3 (JP 2013-091676 A) discloses a UV-curable resin composition containing 30 parts by mass to 95 parts by mass of a polyfunctional epoxy resin, and 5 parts by mass to 70 parts by mass of a monofunctional epoxy compound having a long-chain hydrocarbon backbone having 7 to 20 carbon atoms (see claim 1). In addition, Literature 3 discloses that the UV-curable resin composition contains a polyfunctional oxetane resin (see claim 2) and contains a photopolymerization initiator, such as an ionic photoacid generation-type initiator (see paragraph 0037).
Literature 4 (WO 2015/005210 A1) discloses a cationically polymerizable composition containing a monofunctional epoxy compound and an energy ray-sensitive cationic polymerization initiator (see claim 1). In addition, Literature 4 discloses that the cationically polymerizable composition may contain an oxetane compound (see claim 6). Further, Patent Literature 4 discloses, as in Example 16 and the like, a composition containing a polyfunctional epoxy compound, a monofunctional epoxy compound, an oxetane compound, and a cationic polymerization initiator.
Literature 5 (JP 2005-225961 A) discloses a canonically photopolymerizable composition containing a diglycidyl ether (a) of polyalkylene glycol-added bisphenol A, a bisphenol-type epoxy compound (b) except the (a), and a photocationic catalyst (see claim 1). In addition, Literature 5 discloses that the composition can be blended with a cationically polytnerizable compound, such as oxetane (see paragraph 0018).
However, each of the cationically photopolymerizable compositions disclosed in Literatures 1 and 2 contains the weakly basic substance or the monofunctional vinyl monomer, and hence after the curing of the cationically photopolymerizable composition, the weakly basic substance or the monofunctional vinyl monomer may bleed out (exude) to the surface of a cured product. Accordingly, adhesive performance by the cured product of the cationically photopolymerizable composition is impaired or the deterioration of the appearance of the cured product occurs in some cases.
In addition, each of the cationically photopolymerizable compositions disclosed in Literatures 3, 4, and 5 contains the polyfunctional oxetane resin or the oxetane compound. Accordingly, immediately after the cationically photopolymerizable composition has been irradiated with, for example, UV light, its viscosity may abruptly increase, and hence the working life of the cationically photopolymerizable composition is not sufficiently obtained in some cases.